The present invention relates to a satellite communications system, and, more particularly, to such a system providing communication among many ground station terminals.
One way to allow a greater number ot people to enjoy the benefits of space technology is to provide a system which permits communication among a large number of inexpensive terminals. Such a trend occurred in the transition from telegraph to telephone. Businessmen once traveled to telegraph stations and relied on specially trained operators to convey messages destined for out of town. Today, much communication is effected from the office using a simple and affordable terminal, namely, the telephone. However, document transfer, for example, has not progressed to such convenience.
Satellites promise to extend our communications capabilities. However, just as the mundane telephone belies a costly and complex system of interconnections, so a simple and economical customer premise terminal (CPT) demands a costly and complex satellite system.
The shift of cost and complexity from terminal to transmission system has far more impact when the latter includes a satellite. Additional complexity usually results in increased bulk and weight, which, in turn, affect launch costs. The effect is more than linear once the limits of most commercial launchers are exceeded.
The shift of complexity to a satellite renders impractical many needed repairs and renovations. The cost in downtime and money of replacing a damaged satellite, especially a complex one, reaches nightmare proportions.
Due to the investment required in a satellite, underutilization, insufficient capacity, and mismatch of satellite capability with traffic demands are very costly. Traffic patterns generally increase and shift over the lifetime of a satellite. However, providing for increasing capacity and changing traffic patterns in a spaceborne system presents unique challenges.
It is recognized that risk distribution and flexibility can be provided by a multiple satellite system. However, the number of "slots" in the geosynchronous orbit, the one most useful for communications satellites, is limited. Furthermore, the individual satellites of a multi-satellite system must involve added complexity to communicate with one another, or else the overall capacity of the system must be severely curtailed.
In order to shift some of the cost and complexity to more accessible ground stations, two-hop communications systems have been proposed. In such systems, signals from origination CPTs are directed by a first satellite to a central ground processing station. The processing station amplifies and/or otherwise reformats the signals and redirects the signals to a second satellite for transmission to destination CPTs.
Much of the cost and complexity of a two-hop system resides in the central ground processing station. This lowers the cost of the satellites and renders much of the system accessible for repairs and upgrades.
On the other hand, such proposed two-hop systems require multiple slots in geosynchronous orbit. The increasing competition for such slots renders such two-hop systems impractical. Also, since there are two satellites, the exposure of the system to satellite failure is doubled. Upon failure of a satellite, two-hop operation is lost, and the system is down or else one-hop communications must be resorted to.
What is needed is a satellite communications system which preserves the advantages of a two-hop system, while avoiding the requirement of multiple orbital slots. Furthermore, such a system should permit viable two-hop operation in the event of a satellite failure.